1. Field of the Invention
During the life of a well, periodic measurements and tests are performed to better understand the quality of a reservoir. Some tests are made at the surface and some are performed down hole by sophisticated tools that are lowered into the wellbore.
This invention involves a method for acquiring formation fluid flow rates and calculating the reservoir deliverability by means of a wireline conveyed tool. The field of this invention relates specifically to, designed down hole tools to measure formation fluid flow rates. In the operation of drilling oil and gas wells, it is desirable to evaluate the reservoir deliverability at a stage early enough to make the best economical decision regarding the disposition of the wellbore. This invention allows for the reservoir flow rates to be determined by an apparatus lowered on a wireline into an uncased or cased borehole. A set of inflatable packers are used to isolate an interval of a formation and a flow rate test is performed. The results obtained during the flow test period are transmitted to the surface whereby calculations and deductions can be made as to the validity of the measurements. This ability to record and interpret data as to the potential flow rate of a reservoir, essentially in real time, is of extreme importance to those engaged in well bore evaluations, completions and reserve determinations.
2. Description of the Prior Art
In the past, representative formation fluid flow rate measurements have been primarily restricted to operations involving the use of drill pipe type methods (Drillstem Tests) or production testing. Attempts have been made to measure flow rates using wireline formation sampling and testing tools for many years. The Formation Tester, as it is well known, is a wireline tool used for measuring inferred formation properties and collecting fluid samples. A variety of tools are available to obtain uncontaminated formation fluid samples by means of isolating the wellbore, collecting a sample and measuring the fluid properties. Based on the fluid test results the sample is recovered in a chamber or rejected into the borehole. In the past, the measuring of formation properties by wireline tools has produced unreliable information on the reservoirs ability to produce fluids and estimate the fluid flow rates as a result of the limited tool capacity and capabilities. The financial benefit of performing fluid flow rate tests using a wireline tool, combined with increased data reliability and accuracy is of immense concern to the oil and gas industry.
The remaining discussion on prior art methods and apparatus will strictly be in regards to down hole wireline operations.
In the past, a pair of packers mounted on a wireline tool were lowered into a borehole to obtain formation fluid samples. Expanding the packers isolated an interval in the borehole from which fluids may be drawn into the tool for analysis. If the formation permitted fluid flow and the fluid was suitable for sampling, collection to sample chamber was performed. An example of such a tool is described in U.S. Pat. No. 4,535,843 entitled "Method and Apparatus for Obtaining Selected Samples of Formation Fluids". The tool described in the '843 patent was used to measure fluid properties and collect samples and was not used to determine reservoir fluid flow rates.
Many of the wireline formation testers utilize a probe assembly which extends through a sealing pad into the formation to isolate the tool sample point from the well bore. These tools are capable of obtaining pressure measurements and if desired a sample of the fluids in communication with the sample point. However, during the drilling process of a well, the drilling fluid will invade a permeable formation causing pressure and fluid distortions. Therefore, to make accurate measurements of the essential parameters, virgin reservoir conditions must be observed by the tool. A tool capable of removing the drilling effects must be used before meaningful data can be obtained. The probe type tester has been used to estimate formation permeability, but due to the shallow depth of investigation during fluid removal the tool has its limitations. Multiple probe modifications have been designed in an attempt to improve the situation (such as the tool described in U.S. Pat. No. 4,860,580 entitled Formation Testing Apparatus and Method). The tool in the '580 patent was intended to predict the nature of the formation connate fluid by the accurate determination of the pressure versus depth gradient between the two probe assemblies. By increasing the distance between the probes, deeper depth of investigation can be achieved. But, this technique is limited due to the small bore hole wall area exposed with the probe tools which affects the fluid extraction rate towards the sample point. This sink point also causes the magnitude of the pressure response between the two probes to decrease with increased probe spacing. Therefore, when one wants to measure high reservoir fluid flow rates it is desirable to use a device which is not a probe type testing tool.
Other formation sampling and testing devices have been implemented such as the apparatus found in U.S. Pat. No. 4,513,612 entitled Multiple Flow Rate Formation Testing Device and Method. The tool described in the '612 patent employs the use of a fluid sampling probe and is restricted to the same limitations as discussed.
Flow control by using a restriction device to allow sampling at a constant pressure or constant flow rate can be used to enhance multi probe permeability determinations and such a sampling tool is illustrated in U.S. Pat. No. 4,936,139 entitled Down Hole Method for Determination of Formation Properties. Since the sampling apparatus in the '139 patent had an objective of measuring formation permeability and extracting uncontaminated samples above bubble point pressures, reservoir deliverability and/or the absolute open flow (AOF) potential of the formation was of no concern.
The apparatus of the present invention is designed to allow a large area of the borehole to be exposed for fluid removal by the use of a set of inflatable packers spaced some distance apart which isolates an interval of the formation. This will reduce the affect of the point source used in probe tools and enhance the fluid flow rate determinations. The tool employs a pump which is used to draw large volumes of fluids to an inlet positioned between the packers and discharges the fluid above the top packer. Utilizing the pump to control flow rate and allowing the formation to produce larger volumes of fluids than known designs, permits the opportunity to determine the reservoir deliverability of the formations tested.
A preferred method for obtaining formation deliverability is by means of wireline testing tools because more complete accurate measurements can be made in a fraction of the time required by current drill pipe techniques. The existing limitations with the probe type testers and the bubble point pressure restriction devices warrant an improved method to determine the reservoir deliverability and/or the absolute open flow (AOF) potential of a reservoir. The present invention allows for formation fluid flow rates to be determined by eliminating some of the known wireline tool limitations.